In the manufacture of continuous cast copper, the copper leaving the casting apparatus is generally immediately hot rolled. When exposed to the atmosphere, the copper oxidizes and accumulates surface scale which consists of a mixture of cuprous (red) and cupric (black) oxides. It must be removed or reduced to its metallic state before the copper can be drawn into commercially acceptable wire because oxides on the rod surface cause premature wear of the drawing dies and other production problems. Additionally oxide inclusions in rod may cause breaks in the rod during the drawing process.
Heretofore, different approaches have been suggested for removing the oxide scale from the surface of copper-based products. It should be mentioned that the term "copper" as used herein is meant to also include copper alloys. Exemplary of the approaches suggested for descaling are: (1) mechanically removing the scale as by sanding, shaving or the like, (2) acid cleaning (or pickling), (3) vapor reduction; and (4) non-acid cleaning or reduction.
For example, U.S. Pat. No. 3,623,532, which issued on Nov. 30, 1971 to Chia, et al. and assigned to the assignees of the present invention, discloses a system whereby acid pickling is used for descaling copper rod by immersing the rod in a dilute aqueous acid solution, e.g., sulfuric acid, citric acid, after the cast rod leaves the rolling mill but before it reaches the coiler. This pickling process utilizes the heat contained in the rod to speed up the chemical reaction which results in the reduction of oxides on the surface of the rod. Under these conditions the copper oxides are removed from the surface by the combination of a physical-chemical process; that is, by breaking the scale because the differences in thermal contraction of the oxides and the copper substrate, causes the oxide to shatter and fall away from the rod; by dissolving the oxides which are soluble in an acid; and by chemically reducing the oxides. Usually, in less than one second, the rod has to be cleaned and cooled from a temperature of about 1000 degrees F. to about ambient temperature. The used acid is then returned in the tank and pumped through a heat exchanger and back to the injectors where it is again applied to rod being cooled and cleaned. To maintain optimal cleaning conditions, the cleaning solution is continually regenerated to maintain the copper content and the acid concentration at a predetermined level. This is accomplished by passing the used solution through an electroplating unit and periodically adding new acid to the system.
The foregoing disclosed pickling process has been used with great success by the assignee of the present invention. However, in an effort to reduce operating costs necessitated by the use of acid resistant materials, to avoid ecological problems associated with waste acid disposal, and to produce a more consistent and better quality product, an alternative approach to acid pickling has been developed.
Other techniques which employ one or more reducing gases or vapors to treat oxidized copper rod are disclosed in U.S. Pat. Nos. 3,546,029; 3,562,025; 3,620,853; and 3,659,830, all issued in the name of C. J. Snyder or C. J. Snyder and others, and assigned to Anaconda Wire and Cable Company. In these patents, it is stated that oxide scale is removed by first exposing the rod to high temperature reducing gases or vapors and thereafter immediately quenching the rod in a cooling bath prior to exposure to the atmosphere.
Although the gaseous reduction approach appears to have some advantages over acid pickling, certain disadvantages are inherent in such systems. For instance, the gases or vapors which are suitable for reducing copper oxides are flammable and poisonous, or both, and therefore require special handling to avoid explosion, asphyxiation, or the like. Local, state and federal emission regulations require almost total elimination of organic emissions making it mandatory to practice conservation and emission control measures that allow operators to collect and account for organic solvents used in industrial processes such as the oxide reduction process described above. In fact one great disadvantage of prior art oxide reduction methods and apparatus is the inability of the operator to control solvent loss thereby making the operator subject to fines and other sanctions by regulatory agencies which oversee industry compliance with emission standards.